Application of a “Semi-Structured” Approach with MODFLOW-USG to Inform Metamodels of Stream Depletion for Decision-Support

Abstract:

Regional MODFLOW models commonly use coarse grids which preclude model application to local management issues (e.g., surface-water depletion by wells) without recourse to labor-intensive inset models. The advent of MODFLOW-USG opens the possibility of applying regional models directly to local problems over large areas.

A “semi-structured” approach (uniform grid within layers, different grids among layers) has been tested using the USGS Lake Michigan Basin (LMB) regional model. The original 20-layer model with uniform 5000-ft spacing was converted to four layers with 500-ft spacing in the top glacial layer (where surface-water features are located) overlying coarser resolution layers representing deeper deposits. The MODFLOW-USG version of the LMB model successfully reproduces regional flow conditions and the refined grid spacing improves the simulated response of surface water to nearby wells. Introduction of streamflow routing increases model power to properly simulate headwater streams.

The semi-structured LMB model is designed to feed a decision-support system based on metamodels. A metamodel is a statistical model founded on a computationally-expensive distributed model. In this case three types of metamodels trained on the LMB semi-structured model were tested (artificial neural networks, bayesian networks, and gradient-boosted regression trees) to evaluate the contribution of surface water to wells in shallow groundwater systems. The metamodels show a strong statistical relation between surface-water configuration and stream depletion in the presence of shallow pumping. Two conceptualizations of stream depletion were tested – one related to source of water to wells (“well-centered”), the other to baseflow reduction (“stream-centered”). Both hold promise that the trained metamodels, incorporating existing geospatial data, allow practical decision support across at least part of the US glacial aquifer system.